Abnormal neural regulation of the heart plays a prominent role in the pathophysiology of cardiac arrhythmias, congestive heart failure and sudden cardiac death. Mechanism of neural dysfunction at the end organ level are poorly understood. The central hypothesis of this renewal application is the intrinsic cardiac ganglia are major targets where sensory neuropeptides and paracrine mediators act to disrupt neural regulation of the heart. Specific Aims 1 3 focus on the neuropeptides substance P (SP) and calcitonin gene-related peptide (CGRP) and their role as mediators in intracardiac reflexes. In Aim 1 we use neuroanatomical techniques to map collateral nerve projection that link the ventricular myocardium with the intrinsic cardiac ganglia. Experiments to evaluate the presence of AP and CGRP in these collaterals are an important element of this Aim. Aim 2 focuses on identifying stimuli that trigger intracardiac reflexes mediated by SP and CGRP and defining mechanisms underlying cardiac responses. The following questions are addressed in studies using the isolated heart preparation and anesthetized guinea pigs: Are SP/CGRP-containing afferents activated by mechanical stimuli as well as chemical stimuli? In Aim 3 a cellular approach is used to evaluate the impact of SP on cholinergic neurotransmission in the intrinsic cardiac ganglia and determine whether SP and CGRP interact in affecting intracardiac neurons. Intracellular recording methods will be used to quantify responses of intracardiac neurons to applied peptides, acetylcholine and vagal stimulation? Do SP and CGRP interact in stimulating intracardiac neurons? If so, does the mechanism of interaction require activation of CGRP receptors? Aim 4 is conceptually similar to the preceding work but focuses on the hypothesis that proadrenomedullin N-terminal 20 peptide (PAMP) and adrenomedullin (ADM) function as paracrine mediators in the intrinsic cardiac ganglia. Intracellular recording techniques and in vivo physiological measurements will be used to address two basic questions. Do PAMP and ADM affect intracardiac neurons of their response to acetylcholine? Do PAMP and ADM inhibit release of SP and CGRP and cardiac afferents? Results from these studies will improve our understanding of neural mechanisms that operate within the heart and their importance in cardiac pathophysiology.